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When it comes to moving, ciliates are spectacular to watch.
Their hair-like cilia beat in waves that steer them around the microcosmos, and that’s
great if you’re trying to get from Point A to Point B. But sometimes you’re not actually
trying to get anywhere.
You’re just trying to be slightly less in the spot you are currently in.
Like these spirostomum, which can grow several millimeters in length.
That size can be an advantage when they’re looking for food or navigating the world around
them, but it also makes them easy targets for predators.
Same for the vorticella, which anchor themselves to substrates and then extend their feeding
heads out into the microcosmos.
It makes them kind of like fishermen, waiting for food to come by, but it also makes them
sitting ducks for predators.
But spirostomum, vorticella, and many other ciliates do have a way of defending themselves
from these attacks: contracting.
Contracting is like very, very effective flinching.
If you accidentally touch a hot burner on the stove, you flinch and pull your hand away
very quickly.
Now imagine if you could not only pull your hand back, but you could actually shrink your
hand down into your arm to minimize the threat of the stove even further.
Well, the microcosmos does not have stoves (though, I guess it could be said that stoves
have a microcosmos—but that’s a story for a different day).
Instead, microbes are caught up in a great game of Marco Polo with predators, responding
to chemical and physical cues to detect possible threats, and responding in their own ways.
Of course, there’s the tried and true chemical defense, which spirostomum do use.
Scientists have documented a predator vomiting up a spirostomum whose toxins didn’t digest
very well.
But while toxins are a great defensive option to have, if you’re a spirostomum or any
other ciliate, you probably don’t want to get to the point where you need to be vomited up.
Plus, even if you’ve managed to evade the same predator again and again like spirostomum
has, there’s plenty of other obstacles to avoid in the microcosmos.
Like poop.
I mean yeah, when you have video of Spirostomum getting pooped on you have to use it.
Anyway...
Whether you are avoiding predators or their poop, contracting is a good response to have.
And these contractions are marvelous.
They are some of the fastest things in nature and not only do the contractions happen quickly,
but the time between when the stimulus is received and the contraction begins is faster
than any action you have ever taken.
Scientists have been observing and documenting different ciliates and their contractile behaviors
for decades now, and it’s not hard to understand their fascination.
For one, the contraction looks a bit like the movement of our own muscles, just much
much faster.
Though it turns out it works much differently.
And it’s also just very easy to get a ciliate to contract.
And that turns out to be an important thing when you want to study something.
Stentors have been a popular contractile ciliate model because all you have to do to get them
from their distinctive trumpet shape to a sphere is poke them with a glass needle.
Now, it’s important to note here that we’re using this word “contraction” like it’s
a uniform behavior done in the same way in all ciliates.
But just like how animals have a lot of different ways to run around, ciliates have a lot of
different ways to contract.
Vorticella, for example don’t contract their whole bodies, they spiralize their stalks,
which coil them towards whatever they’re attached to, and away from whatever stimuli
has set them off.
And spirostomum have their own twist—a literal one.
Specialized membrane structures and a long coil of microtubles spin their ends, while
keeping their middle stable to activate contraction in an ultra-fast movement that was only really
deeply understood in 2019.
Now again, whatever the change is for the organism though, one of the things these species
share in common is that the contractions are very, very fast—so much so that it’s taken
high speed cinematography for scientists to start to figure out how it works.
Stentors, spirostomum, and vorticella all condense to fractions of their original length
in a matter of milliseconds.
A contracting spirostomum can experience forces of around 14g.
For comparison, an elite fighter pilot doesn’t ever experience more than 9 gs of force.
And for vorticella, the rapid contraction of their stalk moves them at about 1200 body
lengths per second.
That is very fast.
That’s the equivalent of a cheetah running a mile in a second.
Now interestingly, while the organisms we’re highlighting today work on the scale of milliseconds
to contract, they are much slower on the recovery, taking more on the order of seconds to extend
back to their original state.
Now this makes sense, as the goal is to do whatever possible to escape being eaten, so
the initial quick movement is more important than the return to our normal activities movement.
But the factors driving this fast contraction and slow relaxation are both physical and chemical.
Now physically, the bodies of these organisms contain a mix of contractile fibers that work
together and against each other to create these movements.
In stentors and spirostomum, you can see these special membrane structures running along
their bodies.
And in the vorticella stalk, you can see this helical, contracting organelle called the
spasmoneme.
Whatever these things look like, you can think of any of these movements not as sudden contractions
of muscle fibers, but instead, as chemicals that change shape in different chemical conditions.
If the solution they are in has a bunch of calcium ions, the structures are small and
tightly coiled, if the solution has a low concentration of calcium ions, they stretch out.
And so, in all of these cases, mechanical stimulation causes these cells to suddenly
and rapidly allow calcium ions into the cell, which causes all these structures to dramatically
and rapidly change shape.
But in order to return to the original shape, the calcium ions have to be slowly and carefully
pumped out of the cytoplasm.
So, calcium ions are used across all of biology to carry information and cause changes in
all kinds of biological systems, including the biological system that is you.
But this reliance on calcium ions is kind of strange.
It’s very common.
But the evolution of these systems relying on calcium is a little weird because calcium
ions can interfere with reactions that are key to our cellular metabolism.
One recent theory to try and reconcile this contradiction is that the rapid, calcium-driven
responses we are looking at right now are part of a much longer evolutionary arc—one
built on the need for ancient cells to repair damage to their membranes in order to prevent
a fatal influx of calcium ions from their surroundings.
The same urgency that could have driven the evolution of this biological first aid kit
might have then been rewired for other rapid sensors.
And if this theory is correct, it means that nature converted a defense against a chemical
into a signaling mechanism that drives everything from the contractions you are watching, to
the action potentials that are helping you understand the words that you are hearing right now.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
We would also like to thank Morning Brew again for sponsoring this episode of Journey to the Microcosmos.
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